In the course of thymocyte differentiation, cells bearing T cell receptors (TCR's) that can interact with host major histocompatibility complex (MHC) molecules are selected for further development, but cells that are strongly self-reactive are eliminated. These events are primarily mediated through the interaction of the TCR complex of thymocytes with MHC molecules on thymic epithelial and bone-marrow derived cells. During this developmental process, thymocytes that express both the CD4 and CD8 cell surface glycoproteins (double positive cells) give rise to mature cells that express either one or the other molecule. T lymphocytes that continue to express CD8 have T cell receptors specific for antigen presented in the context of class I MHC molecules, while those that express CD4 have TCR's reactive with antigen and class II MHC molecules. Recent experiments have shown that the CD4 and CD8 molecules can directly interact with MHC molecules. In addition, there is emerging evidence that CD4 and CD8 have signal transducing functions. Results with transgenic mice bearing either class I or class II MHC specific TCR transgenes suggest that CD4 and CD8 have important functions in determining the fate of thymocytes during positive and negative selection. Taken together, these studies suggest that the critical cell-cell interaction events during thymocyte development require the formation of a complex consisting of the T cell receptor, CD4 or CD8, and the MHC molecule on the antigen-presenting cell. The experiments that we propose to carry out will test this model by studying the molecular basis of the interactions of CD4 and CD8 with MHC molecules and, potentially, with the T cell receptor. The importance of these interactions in development will then be examined in mice that express class I MHC transgenes whose products cannot bind to CD8. Analysis of the TCR repertoire in these animals may provide important insight into the role of the CD8-class I interaction in positive selection and thymic tolerance.